The present invention relates to material comminuting machines, and particularly to a shredding machine including improved devices for protecting against damage caused by mechanical shock or overloading of the apparatus.
It is well known for shredding machines to include rotating shafts carrying intermeshed cutters which act against one another to shear material into smaller pieces. Typically, such apparatus includes a cutter box housing the cutters, which are mounted fixedly on counterrotating parallel shafts oriented horizontally, with a feed hopper being located above the cutter box. Material to be comminuted is placed into the feed hopper, where gravity forces it into the proper location to be engaged by the cutters and be torn or cut into small pieces. Frequently, when shredding industrial waste materials such as metal scrap, or when comminuting used automobile tires and the like, the cutting mechanisms are obstructed, at least temporarily, when material too tough or large to be shredded, or in too great a quantity at any one time, becomes lodged between opposing ones of the cutters. If a hard object is the cause of blockage, a significant mechanical shock can be received by the driving shafts and reduction gears of the mechanism, potentially damaging or weakening the drive line of the apparatus.
When such shredders are driven by an electric motor, the electrical current drawn by the motor is often monitored, and the motor is automatically shut down in the case of excessive current, as when the mechanism is blocked, in order to protect against burnout. Devices such as shear pins have been used to protect such shredders and their drive lines against mechanical shock which might otherwise apply excessive amounts of torque to portions of the mechanism.
A common first step in clearing a blockage of the shredding machine is to reverse the rotation of the cutter shafts temporarily in order to provide an additional chance to cut through the material. Repeated attempts to clear blockages in this way may result in cumulative damaging effects from numerous short periods of electrical overloading of the motor. Additionally, if the blockage is caused by material which abruptly stops the cutter mechanism, there may be excessive torque felt by some parts of the mechanism.
Operation of a shredding machine to shred materials which provide frequent mechanical shocks may impose loads greater than the normally-experienced loads, yet of short enough duration that an electrical overload prevention device associated with the motor power supply does not shut down the shredder. This may eventually result in overheating and damage to the motor windings, resulting in expensive down-time for the shredding machine.
What is needed, therefore, is an improved protective device for preventing a shredding machine from damaging itself during extended periods of operation while shredding materials which apply heavy stress to the shredding mechanisms. Preferably, such a protective device could be used to complement electrical overload protection in a shredder driven by an electric motor, and to provide protection against damage of a torque-limiting coupling in a shredding machine driven by either an electric motor or another motor, such as a hydraulic motor.
While it is possible to incorporate a torque-limiting device in the drive line of such a shredding machine, devices used to protect against current overloads in the motor driving such a shredding machine normally allow a high current to pass for a certain amount of time, which may be enough to result in significant damage to a slipping torque-limiting coupling.
Such a torque-limiting coupling typically is adjustable compensate for wear resulting from occasional slippage. Nevertheless, excessive and continuous slippage can result in rapid wear of the friction lining material, and may damage the material irreparably.